1. Field of the Invention
This invention relates to the art and science of ionizing radiation dosimetry. In particular, to the estimation of doses to internal body organs in the digestive tract due to the passage of a solid particle carrying radionuclides which decay by emitting low penetrating ionizing radiations i.e., beta and/or alpha particles, low energy (soft) x-rays and/or gamma rays, and/or Auger electrons!. In many cases these ionizing radiations are absorbed within the dimensions of a body organ. Such solid particles are generally called "radioactive particles". Such particles are typically of the dimensions of micrometers; they normally enter the body by inhalation and/or ingestion. Such particles can be insoluble or only slightly soluble in body fluids. These solid particles can be radioactive in themselves, or may carry radioactive matter with them. The radioactive matter may be soluble or partially soluble in body fluids.
Current dosimetry models adequately treat doses due to the dissolved portion of the radioactive material. This invention pertains to methods for estimating doses in the digestive system due to the passage of the undissolved portion of radioactive material in a solid particle. An incident has occurred in the nuclear industry in which a worker has ingested or inhaled a solid radioactive particle of the type described herein, so this matter is of practical and regulatory import.
2. Description of the Prior Art and Its Limitations
The current internal dose calculational methods (models) of the International Commission on Radiological Protection (ICRP), as employed in its publication ICRP No. 30, for example, calculate the ingestion dose to the mucus layer in the digestive tract, employing a dosimetry model, in which it is assumed that numerous solid radioactive particles are uniformly mixed in the inert matter in the digestive system, or portions thereof. The effective decay energy of a radioisotope is calculated using the effective half-life of the radioisotope in the organ of concern. The effective half-life incorporates the physical half-life of the radioisotope and a biological half-life of the (assumed, many) insoluble, or partly soluble, radioactive particles in the organ.
The ICRP modeling method does not apply to the case of the passage of a single solid particle, or a small group of particles--a particle is either in an organ, or it is not. In such a model, or method, the speeds and positions of an insoluble particle travelling through the digestive tract are not considered explicitly. Moreover, the ICRP method models the dose to the inert mucus layer of the digestive tract, rather than to living tissue.
The current ICRP No. 30 method has been used to set Federal Regulations and Standards governing the safety of workers and the public, e.g., 10 CFR Part 20. But it does not apply to the case of the passage of a single, solid radioactive particle through the digestive system.
Also, the ICRP No. 30 model does not incorporate the probability of dose. The probability of dose depends on the dimensions and composition of the solid radioactive particle, the energies of the radioactive emissions, and the time dependent probabilities of the location and velocity of a solid particle in the digestive system.